EPON is a new type of optical fiber access network technology, which employs passive optical transmission with a point-to-multipoint structure to provide a variety of services on the Ethernet. The EPON is comprised of an Optical Line Terminal (OLT) at the local side, an Optical Network Unit (ONU) at the user side and an Optical Distribution Network (ODN). The EPON uses the PON technology on physical layer and the Ethernet protocol on link layer, and utilizes the topology structure of the PON to realize the access to the Ethernet. Therefore, the EPON integrates the following advantages of the PON and the Ethernet: low cost, high bandwidth, strong expansibility, flexibly and fast service recombination, compatibility with the current Ethernet and convenient management etc.
Optical signals are transmitted between the OLT and the ONU, and there are no active devices on the signal channels, the internal elements include: optical fibers, a passive combiner, and a passive optical coupler/passive optical splitter. The downlink direction is from the OLT to the ONU, i.e. point-to-multipoint, for which the broadcast mechanism is employed. The optical signals sent by the OLT is split onto multiple optical fibers by the passive optical splitter and transmitted to each ONU. The ONU extracts the useful optical signals according to the address of the Media Access Control (MAC) layer. The uplink direction is from the ONU to the OLT, i.e. multipoint-to-point, for which the time division multiplexing mechanism is employed. Optical signals from each ONU are combined onto one optical fiber by the passive optical coupler and transmitted to the OLT. The OLT distributes a time slot for each ONU to avoid collision of data sent by different ONUs. The ONU first caches the data packet, and then sends the cached data packet at the arrival of the time slot belonging to itself.
Each ONU in the EPON utilizes the time division method to access the system. Therefore, only if the OLT and the ONU should be synchronous before the beginning of communication, the correct transmission of information can be ensured. The system synchronization of the EPON is realized by synchronizing a Multi-Point Control Protocol (MPCP) counter which is a local clock counter used for counting time granules. Every frames transmitted by the OLT on the downlink include the value of the MPCP counter. The ONU overlaps the value of the local MPCP counter according to the received value of the MPCP counter. The system synchronization of the EPON requires an information bit sent by the OLT at the OLT local time T. The ONU must receive the information bit at the ONU local time T. Since the distances from each ONU to the OLT in the EPON are different, the transmission delays are different as well. In order to achieve the system synchronization, the ONU clock must have a time delay from the OLT clock, wherein the time delay is a Downlink Delay (DD), that is, if the OLT sends an information bit at timing 0 of the OLT clock, the ONU must receive the information bit at timing 0 of the ONU clock. The Round-Trip Time (RTT) is the sum of the DD and an Uplink Delay (UD). The RTT must be known and transmitted to the ONU by the OLT. The process of obtaining the RTT is called ranging. The data sent by different ONUs on the downlink of the same OLT will not collide only when the system synchronization of the EPON is realized.
The typical topology structure of the EPON system is in a tree form, as shown in FIG. 1. A fault occurs at the trunk optical fiber between the OLT and the optical splitter will result in communication faults of all ONUs on the downlink of the OLT simultaneously. Therefore, for the important areas and important sites, protection of the trunk optical fiber to improve the reliability of the whole PON system is of great importance. A block diagram for implementing trunk optical fiber protection currently utilized by the EPON is shown in FIG. 2. Protecting the trunk optical fiber primarily comprises: setting a redundant trunk optical fiber, i.e. a standby trunk optical fiber, of a different path. Switch between the master trunk optical fiber and the standby trunk optical fiber is performed when a fault occurs at the master trunk optical fiber. Since the path lengths of the master trunk optical fiber and the standby trunk optical fiber are usually different, the UD, DD and RTT will change after the switch. The ONU updates the local MPCP counter in real time according to the received time stamp identifier which is from the OLT. Since the path lengths of the master trunk optical fiber and the standby trunk optical fiber are different, the MPCP counter of the ONU usually has a change of more than 8 Time Quantas (TQs). However, if the change of the MPCP counter of the ONU is more than 8 TQs, it will result in the ONU disconnection and requiring of a re-registration. Thus, the service restoration time cannot meet the carrier-grade requirement of 50 ms. Specifically, the TQ is a time unit of PON, with 1 TQ being equal to 16 ns.
It can be seen from the aforementioned description that, how to avoid re-registration of the ONU during protection switch of the EPON so as to shorten the service restoration time has become a problem to be solved.